The invention relates to an ultra-high frequency solid-state oscillator stabilized by using an injection locking method, and more particularly to an injection-locked ultra-high frequency solid-state oscillator employing an optional low-frequency injection signal and having wide locking bandwidth.
As for the technique to stabilize and ultra-high frequency solid-state oscillator, such as an IMPATT diode oscillator or a Gunn diode one, two methods have been developed so far. They are the high-Q resonator loading method and the injection locking method:
In the high-Q resonator loading method, noise is reduced by increasing Q of a resonator which is a part of the oscillator. In this method, however, the frequency of the oscillation output cannot be changed otherwise than by mechanically changing the resonant frequency of the high-Q resonator; the oscillator thus stabilized is lacking in the so-called electrical tunability.
Meanwhile, the injection locking method has conventionally been classified roughly into three method. That is, a first method is called a fundamental-wave injection locking method. As shown in FIG. 1, this method is to inject, through a circulator 2 an output power with frequency f.sub.inj from an oscillator 3 into the main solid-state oscillator 1 running freely at frequency f.sub.o. The frequency f.sub.inj of the injection signal may be equal to or very close to the frequency f.sub.o of the main oscillator 1. Here, if the following equation (1) is satisfied between the output power P.sub.o of the main oscillator 1 and the injection signal power P.sub.inj of the oscillator 3, the locked state is established and the oscillation frequency f.sub.o of the main oscillator 1 will coincide with the injection signal frequency f.sub.inj, ##EQU1## Here Q.sub.ext is a coupling coefficient between the main oscillator 1 and the external circuit.
In this locking method, upon completion of locking, the main oscillator 1 produces an output power with reduced noise if the injection signal source 3 is stable with relatively low noise. Thus the output power of the oscillator 3 has to have a high frequency-stability. In general, however, as the oscillation frequency increases, it becomes more difficult to realize the oscillator with low noise. Therefore in this method, it is difficult to realize an oscillator which can provide low-noise and stable injection signal at frequency equal or very close to ultra-high frequency solid-state oscillator to be stabilized.
The second injection locking method is called a subharmonic injection locking one. In this method, use is made of the injection signal source 3 whose frequency f.sub.inj is nearly equal to 1/n of the oscillation frequency f.sub.o of the main ultra-high frequency solid-state oscillator 1 ##EQU2## here n = 2, 3, 4 . . . ). The injected signal frequency is multiplied by nth order due to the nonlinearity of the oscillating element in the main oscillator 1, thereby producing a signal with frequency of f.sub.o + .DELTA.f. Here, if the following equation (2) is satisfied between the output power P.sub.o of the main oscillator 1 and the injection power P.sub.inj from the injection signal source 3, the locked state is established and the oscillation frequency f.sub.o of the main oscillator 1 coincides with frequency f.sub.o + f which is n times the injection signal frequency f.sub.inj. EQU .vertline.f/f.sub.o .vertline. .ltoreq. k (P.sub.inj /P.sub.o).sup.n/2 . . . . . (2)
here k is a coupling coefficient between the main oscillator 1 and the external circuit.
According to this second locking method, a low-frequency oscillator may be used satisfactorily as the injection signal source. Such low-frequency oscillator can be realized relatively easily. However, this method has a defect that as the order of multiplication (n .apprxeq. f.sub.o /f.sub.inj) is increased, the locking bandwidth is reduced. Here the locking bandwidth means a frequency range within which the frequency of the main oscillator 1 can vary in obedience to the variation of the frequency of the injection signal. According to an experimental example, it is recognized that if n is 9, subharmonic injection locking method is applied to the oscillator operating at 8.5 GHz, and the locking bandwidth .DELTA.f is reduced substantially to nearly 1 MHz. On the other hand, locking bandwidth .DELTA.f of the fundamental-wave injection locking method is about 100 MHz or more. This means that in the second method, the oscillation-wave frequency f.sub.o can vary within a very narrow frequency range.
The third method is called a sideband-wave injection locking method. In this method, a low-frequency signal with frequency f.sub.inj,1 is injected into the main oscillator 1, thereby producing sideband waves with frequencies f.sub.U = f.sub.o + f.sub.inj' 1 and f.sub.L = f.sub.o - f.sub.inj,1 near the oscillation-wave frequency f.sub.o. Another injection signal with a frequency f.sub.inj,2 near the frequency f.sub.U or f.sub.L is injected into the oscillator 1. One of the side-band wave frequencies, f.sub.U or f.sub.L is, then, locked to the frequency f.sub.inj,2 according to the fundamental-wave injection locking method. Thus, the frequency f.sub.o of the main oscillation wave itself is stabilized and its sideband noise is reduced, while by fixing the frequency of one of the two injection-wave signal and by changing the frequency of the other, the frequency f.sub.o of the main oscillation wave can be made variable in accordance with the relation f.sub.o + f.sub.inj,1 = f.sub.inj,2 or f.sub.o - f.sub.inj,1 = f.sub.inj,2.
In this method, however, the frequency f.sub.inj,2 of the second injection signal must be as high as the main oscillation-wave frequency f.sub.o of the solid-state oscillator 1. It is, therefore, difficult to realize the second injection signal source.
Accordingly, an object of this invention is to provide an injection-locked ultra-high frequency solid-state oscillator capable of using a low-frequency injection signal source, having wide locking bandwidth and having electrical tunability.